It is well known that the crosstalk performance of twisted pair cables is affected by the lay lengths of the pairs within the cable assembly. As one example of twisted pair cable designs, Category rated network cables have become more complex and have some of the highest crosstalk expectations. To provide the ability to maintain the high level of performance, it is desirable to have the ability to measure the near instantaneous lay of each wire pair. Also, due to unavoidable processing variations along the pairing, payoff, and cable assembly operations, it is also desirable to be able to measure the pair lay as the pair enters the cable assembly and becomes relatively fixed in that assembly.
It is known that the wire pairs may be subjected to stretch or rotational forces during pair assembly and during the assembly of separate pairs in to a cable. These forces have a significant effect on the pair lay. Pair rotation during the spooling and guiding of the pair in to the cable assembly causes local increases or decrease in pair lay, even if the pair was manufactured with exact and uniform lay in a prior process. Stretching can be systematic or erratic during the cable assembly process and it is desirable to monitor that effect.
It is also known that multiple (perhaps 10 to 100) pairing machines may feed a single cabling machine that assembles the pairs. However, subtle differences may exist in pair lay from machine to machine, even if the separate pairing machines have the same pair lay set point. This paring machine variation is one of the key difficulties that exist in making multiple pair cables in large quantities with consistent crosstalk performance.
A need exists for a measuring device that allows lays to be measured as well as lay ratios during the assembly of completed cable. Some cable designs may require pairing machines to deliberately change the pair lay along the length of the cable, such as described in U.S. Pat. No. 7,375,284. It is of interest to measure these varying lay lengths at subsequent process operations to ensure that the design values are actually implemented. Pair lay measurement designs that do not specifically measure such variation miss critical and unavoidable causes of crosstalk performance variation in finished cables.
To date there has been no practical way to measure pair lay length values along a length of cable. It is highly desirable to provide this information and correlate to subsequent off-line measurements.